Many methods for treating liquids with magnetic particles are known in the prior art; these usually relate to the separation of nucleic acids or other biologically or biochemically relevant substances from a solution.
Methods which are based on magnetic separation by using specifically and/or nonspecifically binding magnetic particles have gained increasing importance in the field of sample preparation for diagnostic or analytical examinations, in particular for the isolation of nucleic acids, proteins and cells.
This applies in particular to automated methods, since in this way a large number of samples can be prepared within a short time and labor-intensive centrifuging steps can be obviated. The requirements for an efficient and high sample throughput are satisfied in this way. This is of enormous importance since purely manual handling of very large sample numbers is practically unfeasible.
The basic principle of the magnetic separation of substances from complex mixtures is based on providing magnetic particles with specific binding properties for the target substances to be separated, for example by chemically treating their surface. The size of such magnetic particles generally lies in the range of from about 0.05 to 500 μm, so that they have a large surface area for the binding reaction. Depending on their size and composition, the magnetic particles may have a density which is close to the density of the liquid in which they are suspended. In this case, sedimentation of the magnetic particles may readily take a few hours.
In known separation methods, the magnetic particles are immobilized at one position by using magnetic forces or a magnetic field, for example by means of a permanent magnet. This accumulation of the magnetic particles is also referred to as pellet or magnet sediment. The liquid supernatant is subsequently removed, for example by suction or pouring off, and discarded. The fact that the magnetic particles are immobilized by the magnetic forces substantially prevents magnetic particles from being removed together with the supernatant.
Typically, the immobilized magnetic particles are subsequently resuspended. In order to enrich the bound target substances, an elution liquid or elution buffer is used. The binding between the target substance and the magnetic particles is broken, and the target substance molecules are released from the magnetic particles. The target substance molecules can then be removed together with the elution liquid, while the magnetic particles are immobilized by the action of a magnetic field. In order to reduce the volume of elution liquid in relation to the primary starting volume for the binding, the target substance molecules may not only be enriched but also concentrated. Before the elution step, one or more washing steps may be carried out.
Various types of devices have been described for carrying out such separation methods by means of magnetic particles. For instance, US 2001/0022948 describes a device in which a magnetic rod is immersed in a first reaction vessel which contains magnetic particles suspended in liquid.
There, the magnetic rod attracts the magnetic particles so that the magnetic particles adhere to the rod. The magnetic rod is then taken out of the reaction vessel, together with the magnetic particles adhering to it, and put into a second reaction vessel. There, the magnetic force of the rod can be reduced or switched off so that the magnetic particles are released from the rod and suspended in a liquid contained in the reaction vessel. Similar methods are also known from in U.S. Pat. No. 6,065,605 and WO 2005/005049.
On the other hand EP 0 965 842 discloses a device in which the magnetic particles, together with the liquid in which they are suspended, are taken up in a pipette. The pipette tip has a special separation region, to which a magnetic field can be applied by using a magnet. The magnetic particles are thereby immobilized as pellet or magnet sediment on the inside of the pipette tip. The aspirated liquid is subsequently removed from the pipette tip by the pipetting function of the device.
The magnetic field in the separation region can subsequently be removed, so that the magnetic particles immobilized in the pellet are released again. A similar method and a similar device are described in U.S. Pat. No. 6,187,270.
Another principle for the separation of magnetic particles is described by EP 015 905 520. In this case, the magnetic particles remain in the same reaction vessel while the liquid in this vessel is replaced. In order to adapt to a particular process step, the magnet sediments can be immobilized at a desired height on the side wall of the reaction vessel. This is done by providing magnets which are respectively arranged at a different distance from the rotation axis on various arms of a rotatably mounted carrier. By rotating the carrier, a particular arm—and therefore a particular magnet—can be brought into the vicinity of the side wall of the reaction vessel. The magnetic particles are then immobilized as pellet at this position.
Said conventional devices and methods all have the common feature that they are configured as so-called “open systems”, since, according to their respective functional principle, magnetic rods or pipettes have to be introduced one or more times into the reaction vessel. These conventional devices and methods therefore entail the risk of cross-contaminating other reaction vessels by aerosol and/or droplet formation. Examination results may be vitiated or even unusable.